1. Field of the Invention
This invention relates to processes of treating metal surfaces with aqueous acidic compositions for forming conversion coatings by drying in place. The invention is particularly suited to treating iron and steel, galvanized iron and steel, zinc and those of its alloys that contain at least 50 atomic percent zinc, and aluminum and its alloys that contain at least 50 atomic percent aluminum.
2. Statement of Related Art
A very wide variety of materials have been taught in the prior art for the general purposes of the present invention, but most of them contain hexavalent chromium which is environmentally undesirable. The specific items of related art believed by the applicant to be most nearly related to the present invention are noted below.
U.S. Pat. No. 4,921,552 of May 1, 1990 to Sander et al. teaches treating aluminum with a composition comprising fluozirconic acid, hydrofluoric acid, and a water soluble polymer.
Published European Patent Application 0 273 698 (published Jul. 6, 1988) teaches aqueous acidic treating solutions comprising trivalent metal compounds, silica, and preferably also nickel and/or fluoride ions. The counter anions for the trivalent metal cations used may be silicofluoride.
South African Patent 85/3265 granted Dec. 24, 1985 teaches treating metal surfaces, including galvanized iron and steel, with an acidic aqueous composition comprising a fluoride containing compound selected from hydrofluoric acid and fluoboric, fluosilicic, fluotitanic, and fluozirconic acids and their salts; one or more salts of a metal such as cobalt, nickel, copper, iron, manganese, strontium, and zinc; and, optionally, a sequestrant and/or a polymer of acrylic acid, methacrylic acid, or esters thereof. Metal surfaces are treated with this composition, then rinsed with water, and preferably are then rinsed with a solution containing chromic acid.
U.S. Pat. No. 4,339,310 of Jul. 13, 1982 to Oda et al. teaches an aqueous chromium free composition comprising a soluble compound of titanium or zirconium which may be fluotitanate or fluozirconate, a pyrazole compound, a myoinositol phosphate ester or a salt thereof, and a silicon compound which may be "silicon hydrofluoride" or "ammonium silicafluoride" as a useful surface treatment for tin cans.
U.S. Pat. 4,273,592 of Jun. 16, 1981 to Kelly teaches an acidic aqueous composition comprising a zirconium or hafnium compound which may be the fluozirconate or fluohafnate, a fluoride compound which may also be the noted complex fluoride compounds, and a polyhydroxy compound having no more than about seven carbon atoms. The composition is substantially free from hexavalent chromium and elements such as boron, manganese, iron, cobalt, nickel, molybdenum, and tungsten and also substantially free from ferricyanide and ferrocyanide.
U.S. Pat. No. 4,148,670 of Apr. 10, 1979 to Kelly teaches treating aluminum with an aqueous composition comprising a zirconium or titanium compound which may be the fluozirconate or fluotitanate, a fluoride compound which may also be the noted complex fluoride compounds, and phosphate ions.
U.S. Pat. No. 3,593,403 of Nov. 10, 1970 to Ries teaches treating galvanized and other zinciferous metal surfaces with aqueous acidic compositions comprising complex fluorides of iron, titanium, zirconium, and/or silicon and at least one oxidizer.
U.S. Pat. No. 3,506,499 of Apr. 14, 1970 to Okada et al. teaches treating aluminum and zinc surfaces with an aqueous solution of chromic acid and colloidal silica.
U.S. Pat. No. 3,160,506 of Dec. 8, 1964 to O'Connor et al. teaches preparing a metal substrate for application of a photographic emulsion by contacting the metal substrate with an aqueous solution containing an acid, alkali metal, or alkaline earth metal salt of a transition metal fluoride and sealing the layer formed thereby by subsequent treatment with chromic acid.
U.S. Pat. No. 3,066,055 of Nov. 27, 1962 to Pimbley teaches treating aluminum surfaces with a composition comprising transition metal cations having atomic numbers from 23-29 inclusive and preferably also comprising hexavalent chromium, molybdate, or tungstate anions and halogen anions, which may be complex fluorides.
U.S. Pat. No. 2,825,697 of Mar. 4, 1958 to Carroll et al. teaches treating aluminum and its alloys with an aqueous composition comprising a fluorine bearing compound which may be fluozirconic, fluosilicic, fluoboric, fluotitanic, or fluostannic acids or their salts together with at least 0.4 grams per liter (hereinafter "g/L") of CrO.sub.3 (or its stoichiometric equivalent of other types of hexavalent chromium).
U.S. Pat. No. 2,276,353 of Mar. 17, 1942 to Thompson teaches treating metals with a combination of fluosilicic acid or its salts and an oxidizing agent.
U.S. Pat. No. 1,710,743 of Apr. 30, 1929 to Pacz teaches treating aluminum with aqueous solutions containing complex fluoride ions and optionally also including cations of silver, nickel, cobalt, zinc, cadmium, antimony, tin, lead, iron, and manganese. The amount of the compounds present containing these heavy metal cations must be substantially less than that of the complex fluoride salts present, with amounts of about one-tenth that of the complex fluoride being noted as excellent.
U.S. Pat. No. 1,638,273 of Aug. 9, 1927 to Pacz teaches treating aluminum surfaces with an aqueous composition comprising a combination of a nickel or cobalt salt, a soluble fluosilicate salt, and an alkali nitrate, phosphate, or sulfate.